For engine systems in vehicular or other mobile applications where a supply of hydrogen is required, due to challenges related to on-board storage of a secondary fuel and the current absence of a hydrogen refueling infrastructure, hydrogen is preferably generated on-board using a fuel processor. The hydrogen-containing gas from the fuel processor can be used to regenerate, desulfate and/or heat engine exhaust after-treatment devices, can be used as a supplemental fuel for the engine, and/or can be used as a fuel for a secondary power source, for example, a fuel cell.
One type of fuel processor is a syngas generator (SGG) that can convert a fuel into a gas stream containing hydrogen (H2) and carbon monoxide (CO), known as syngas. Air and/or a portion of the engine exhaust stream can be used as an oxidant for the fuel conversion process. The exhaust stream typically contains oxygen (O2), water (H2O), carbon dioxide (CO2), nitrogen (N2) and sensible heat, which can be useful for the production of syngas. Steam and/or water can optionally be added. The fuel supplied to the SGG can conveniently be chosen to be the same fuel that is used in the engine. Alternatively a different fuel can be used, although this would generally require a separate secondary fuel source and supply system specifically for the SGG. The H2 and CO can be beneficial in processes used to regenerate exhaust after-treatment devices. For other applications, for example, use as a fuel in a fuel cell, the syngas stream may require additional processing prior to use.
In vehicular or other mobile applications, an on-board SGG should generally be low cost, compact, light-weight, of low power consumption and efficiently packaged with other components of the engine system. Some particular challenges associated with the design of fuel processors used in engine systems to convert a fuel and engine exhaust gas stream into a hydrogen-containing stream include the following:                (a) Engine exhaust stream output parameters, such as mass flow, pressure, temperature, composition and emission levels, vary significantly over the operating range of the engine.        (b) The output required from the fuel processor is typically variable. The hydrogen-containing gas stream is preferably generated as-needed in accordance with the variable demand from the hydrogen-consuming devices. This reduces the requirement for additional storage and control devices.        (c) High system reliability and durability are typically required.        (d) The fuel processor should be capable of operating over a wide ambient temperature range, different elevations and under a range of other external and environmental conditions.        
The thermochemical conversion of a hydrocarbon fuel to syngas is performed in a SGG at high operating temperatures with or without the presence of a suitable catalyst. Parameters including, equivalence ratio, oxidant-to-carbon ratio, steam-to-carbon ratio, and operating (reaction) temperature are typically adjusted in an attempt to increase the efficiency of the fuel conversion process while reducing the undesirable formation of carbon (coke or soot), which can cause undesirable effects within the SGG and/or on downstream components.
Typically, a high SGG operating temperature is desired in order to increase the fuel conversion efficiency of the process and to reduce the size of the SGG. However, excessive operating temperatures can cause undesirable effects including catalyst sintering, formation of loose, amorphous soot and the requirement for the use of thermally robust specialty materials. Insufficient operating temperatures can cause undesirable effects including reduced chemical kinetics, reduced stability of a reaction flame, low fuel conversion, high concentrations of unconverted hydrocarbons in the product syngas, and formation of dense, more graphitic carbon or coke. In conventional SGGs, thermal insulation is often employed to reduce the amount of heat lost from the fuel processor.
An increase in fuel conversion of the SGG can enable a desirable reduction in volume of the SGG for a given syngas output. This reduction in volume can however result in a decrease in the surface area of the SGG, reducing the heat transfer capacity from the SGG to the ambient environment. During some operating conditions of a SGG, a passive method of dissipating waste heat can be insufficient to prevent excessive operating temperatures.
The present fuel processor with improved design, components and operating methods is effective in addressing at least some of the issues discussed above, both in engine system applications and in other fuel processor applications.